More specifically, the invention relates to a process for improving the adhesion of particles of metals, such as platinum, to a carbon substrate, the supported metals being used, in particular, for manufacturing the electrodes of a membrane electrolysis cell having a cathode for the reduction of oxygen, which produces a solution of an alkali metal hydroxide and of chlorine by the electrolysis of an aqueous alkali-metal chloride (NaCl or KCl) solution.
Such an electrolysis cell generally consists of a cation exchange membrane which divides the cell into an anode compartment and at least one cathode compartment in which the cathode is placed, the compartment being supplied with a gas containing oxygen.
The anode and cathode reactions in such electrolytic processes using a cathode for the reduction of oxygen are:
at the anode: 2Cl.sup.-.fwdarw.Cl.sub.2 +2e.sup.- PA1 at the cathode: H.sub.2 O+1/2O.sub.2 +2e.sup.-.fwdarw.2OH.sup.-
In order to speed up the reaction occurring at the cathode and lower the absolute value of the reaction overpotential these are employed, catalysts, generally noble metals such as platinum, silver or gold deposited on electrically conducting supports, such as in particular carbon materials having a high specific surface area.
Preferably, platinum or silver, and more particularly platinum, is used since, although this is an expensive metal, it is the one for which the cathodic overpotential is lowest in absolute value.
It has been found in tests on the stability of catalysts based on platinum deposited on carbon in hot concentrated sodium hydroxide under oxygen for several hours that there is a significant loss of platinum.
This significant loss of platinum results in a deterioration in the electrochemical performance, such as an increase in the cell voltage.
This loss of platinum from the catalyst used in the electrodes of membrane electrolysis cells having a cathode for the reduction of oxygen has been mentioned by various authors.
Thus, S. Kohda et al. in Soda to Enso, 1995, Volume 46(10), pages 402 to 419 (Absc. Number: 124-129655) report that, after a continuous operation of 175 days, analysis of the electrodes indicates a considerable loss of platinum. This loss, according to these authors, is influenced by the operating conditions and in particular by the electric current.
In order to limit the loss of platinum, these authors have proposed using platinum-iridium as the catalyst.
Kagaku Koggakukai in Soda to Enso, 1996, Volume 47(1), pages 16 to 31 points out that, in the case of air cathodes in a basic medium (NaOH solution containing 9 mol/l of NaOH) at 85.degree. C., there is a loss of platinum and a deterioration of the carbon support.